linux-kernel - [PATCH tip/core/rcu 18/22] rcu: Remove CPU-hotplug failsafe from force-quiescent-state code path

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Date:   Tue, 26 Jun 2018 10:10:44 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     linux-kernel@...r.kernel.org
Cc:     mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 18/22] rcu: Remove CPU-hotplug failsafe from force-quiescent-state code path

Now that quiescent states for newly offlined CPUs are reported either
when that CPU goes offline or at the end of grace-period initialization,
the CPU-hotplug failsafe in the force-quiescent-state code path is no
longer needed.

This commit therefore removes this failsafe.

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 include/trace/events/rcu.h | 10 +++++-----
 kernel/rcu/tree.c          |  9 +--------
 kernel/rcu/tree.h          |  1 -
 3 files changed, 6 insertions(+), 14 deletions(-)

diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
index 759e7e83733d..a8d07feff6a0 100644
--- a/include/trace/events/rcu.h
+++ b/include/trace/events/rcu.h
@@ -391,11 +391,11 @@ TRACE_EVENT(rcu_quiescent_state_report,
 
 /*
  * Tracepoint for quiescent states detected by force_quiescent_state().
- * These trace events include the type of RCU, the grace-period number that
- * was blocked by the CPU, the CPU itself, and the type of quiescent state,
- * which can be "dti" for dyntick-idle mode, "ofl" for CPU offline, "kick"
- * when kicking a CPU that has been in dyntick-idle mode for too long, or
- * "rqc" if the CPU got a quiescent state via its rcu_qs_ctr.
+ * These trace events include the type of RCU, the grace-period number
+ * that was blocked by the CPU, the CPU itself, and the type of quiescent
+ * state, which can be "dti" for dyntick-idle mode, "kick" when kicking
+ * a CPU that has been in dyntick-idle mode for too long, or "rqc" if the
+ * CPU got a quiescent state via its rcu_qs_ctr.
  */
 TRACE_EVENT(rcu_fqs,
 
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 8704b31b696d..c698a595b932 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1188,14 +1188,6 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
 		smp_store_release(ruqp, true);
 	}
 
-	/* Check for the CPU being offline. */
-	if (!(rdp->grpmask & rcu_rnp_online_cpus(rnp))) {
-		trace_rcu_fqs(rdp->rsp->name, rdp->gp_seq, rdp->cpu, TPS("ofl"));
-		rdp->offline_fqs++;
-		rcu_gpnum_ovf(rnp, rdp);
-		return 1;
-	}
-
 	/*
 	 * A CPU running for an extended time within the kernel can
 	 * delay RCU grace periods.  When the CPU is in NO_HZ_FULL mode,
@@ -3716,6 +3708,7 @@ void rcu_cpu_starting(unsigned int cpu)
 		nbits = bitmap_weight(&oldmask, BITS_PER_LONG);
 		/* Allow lockless access for expedited grace periods. */
 		smp_store_release(&rsp->ncpus, rsp->ncpus + nbits); /* ^^^ */
+		rcu_gpnum_ovf(rnp, rdp); /* Offline-induced counter wrap? */
 		if (rnp->qsmask & mask) { /* RCU waiting on incoming CPU? */
 			/* Report QS -after- changing ->qsmaskinitnext! */
 			rcu_report_qs_rnp(mask, rsp, rnp, rnp->gp_seq, flags);
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 6683da6e4ecc..795d469c6f67 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -217,7 +217,6 @@ struct rcu_data {
 
 	/* 4) reasons this CPU needed to be kicked by force_quiescent_state */
 	unsigned long dynticks_fqs;	/* Kicked due to dynticks idle. */
-	unsigned long offline_fqs;	/* Kicked due to being offline. */
 	unsigned long cond_resched_completed;
 					/* Grace period that needs help */
 					/*  from cond_resched(). */
-- 
2.17.1